1. Field of the Invention
The present invention relates to a process for producing an oxide magnetic material having soft magnetism, particularly Mn--Zn ferrite. More particularly, the invention relates to a process for producing Mn--Zn ferrite which enables scraps of a sintered product to be regenerated and reused.
2. Description of the Related Art
There is Mn--Zn ferrite as a representative oxide magnetic material having soft magnetism. This Mn--Zn ferrite has conventionally widely been used as low loss materials used in switching power supply transformer, flyback transformer or deflection yoke, various inductance elements, impedance elements for EMI countermeasure, electromagnetic wave absorbers and the like. This Mn--Zn ferrite generally has a composition comprising basic components of more than 50 mol % of Fe.sub.2 O.sub.3, 52 to 55 mol % of Fe.sub.2 O.sub.3 on the average, 10 to 24 mol % of ZnO and the remainder being MnO. The Mn--Zn ferrite has conventionally been produced by mixing each raw material powder of Fe.sub.2 O.sub.3, ZnO and MnO in predetermined proportion, forming the resulting mixture into a predetermined shape through each step of calcination, milling, component adjustment, granulation, pressing and the like, and then subjecting a green compact to sintering treatment such that the green compact is maintained at 1,200 to 1,400.degree. C. for 3 to 4 hours in a reducing atmosphere having oxygen concentration greatly decreased by letting nitrogen gas flow.
The reason for sintering in a reducing atmosphere is that since the green compact contains Fe.sub.2 O.sub.3 in a large amount of 50 mol % or more, if it is sintered in the air, densification does not proceed sufficiently, and as a result, good soft magnetism is not obtained. Further, Fe.sup.2+ to be formed by reduction of Fe.sup.3+ has a positive crystal magnetic anisotropy, and therefore has the effect that it offsets a negative crystal magnetic anisotropy of Fe.sup.3+, thereby increasing soft magnetism. However, if sintered in the air, formation of Fe.sup.2+ by such a reduction reaction cannot be expected. In the above described milling step, milling is conducted such that powder has an average particle size of about 1.0 to 1.4 .mu.m. The reason for this is that if the average particle size is larger than 1.4 .mu.m, the desired density is not obtained in sintering, and if the average particle size is smaller than 1.0 .mu.m, it is difficult to handle the powder.
In the production of Mn--Zn ferrite as described above, many scraps are produced in each step for the reasons of a surplus of ferrite, defective ferrite or the like. Wastes produced in the steps before pressing have no specific problem on its regeneration utilization. However, regarding scraps of a sintered product due to defects such as dimensional defect, crack, breakage or the like in the sintering step, it is the trend that it is difficult to regenerate and reuse those for the reasons described hereinafter and those are disposed.
The reason why regeneration utilization of scraps of a sintered product is difficult is explained below.
The sintering step of Mn--Zn ferrite is rate-determined to a vacancy concentration of oxygen ion that has the slowest diffusion rate in the constituent ions. Factors which govern this are a content of Fe.sub.2 O.sub.3 and an oxygen concentration in an atmosphere. Vacancy of oxygen ion tends to be easily formed as the Fe.sub.2 O.sub.3 content is small and the oxygen concentration in an atmosphere is low. However, since the conventional Mn--Zn ferrite contains Fe.sub.2 O.sub.3 in an amount of more than 50 mol %, vacancy of iron ion, manganese ion and zinc ion is largely formed corresponding to decrease in vacancy of oxygen ion. In other words, if it is intended to mill and press a sintered product of the conventional Mn--Zn ferrite for reuse, sintering must be conducted under the condition that oxygen concentration in an atmosphere is considerably lowered. However, the oxygen concentration which can be lowered in the actual mass-production step is at most about 0.1%, and the oxygen concentration in this degree can not secure the necessary vacancy concentration of oxygen ion. As a result, sintering does not proceed smoothly, making it difficult to obtain the desirable density of the ferrite.